Bisulphite pulping of pine wood



United States Patent 3,210,237 BISULPHITE PULPING 0F PINE WOOD Elmo H. Madison, International Falls, Minn., assignor, by

mesne assignments, to Boise Cascade Corporation, Minneapolis, Minn., a corporation of Delaware Filed Apr. 15, 1963, Ser. No. 273,215 8 Claims. (Cl. 162-61) This invention relates to an improved pulp and a process for pulping wood chips and more particularly to a pulping process for pine in which the pulping liquor is a bisulphite solution.

The present invention is directed to a pulp and the process of manufacturing cellulosic pulp by an improved sulphite cooking process employing a relatively pure magnesium base cooking liquor. The process of this invention permits the use of higher temperatures and the obtaining of a brighter pulp from pinewood chips in less cooking time.

Heretofore the largest part of sulphite pulp has been made from spruce wood. Because of the enormous increase in the production of pulp the time has come in many places where there is not enough spruce which can be obtained economically from a native forest to meet the demands. Attempts have been made for a long time to use other soft woods for the production of sulphite pulp. The interest in pine has been particularly great. It is known that various parts of pine, notably the heartwood and sapwood, behave quite differently toward ordinary sulphite cooking liquors. As a result pine wood is not very popular in making sulphite pulp. Some have attributed the behavior of the pine in this regard to its high resin content. The difiiculty may also be assigned to the substance known as 3,5-dihydroxystilbene and 3 hydroxy-S methoxystilbene which are present in the pine heartwood. The first of these is called pinosylvin and the second would then be pinosylvin monornethylether. These stilbenes, under normal cooking conditions are believed to react at higher acidities and higher temperatures with lignin in the chips in such a way as to prevent the lignin from reacting properly with the cooking liquor and be dissolved out of the chips to the degree desired.

An object of the invention is in general to provide an improved pulping process as well as pulp produced thereby and capable of using the various types of pine and particularly jack pine. It has been found in accordance with this invention that a pulp having desirable properties can be produced by cooking chips with a solution of bisulphite such as magnesium. The nature of the cooking liquor used in accordance with this invention, and the essential distinction between it and conventional liquors will be apparent from the following explanation.

In conventional sulphite processing the sulphur dioxide content is reported in terms of free S0 and combined S0 In fact the amount of sulphur dioxide reported as combined is the amount theoretically combined with the base as a monosulphite. The amount of sulphur diox ide reported as free S0 will include a percentage of sulphur dioxide which is actually in combination to form the bisulphite and some true free sulphur dioxide. The amount of free sulphur dioxide which is in combination to form the bisulphite will be equal to the sulphur dioxide reported as combined and a true free sulphur dioxide can be calculated by subtracting the value of the combined sulphur dioxide from the value of the free sulphur dioxide.

It has been found by employing the magnesum base at the bisulphite stage, that is liquor containing mainly magnesium bisulphite and only minor amounts of monosulphite or else minor amounts of true free sulphur dioxide, it is possible to obtain solutions of adequate concentration for the digesting of wood. With such cooking Patented Oct. 5, 1965 ice liquor using proper cooking conditions it has been possible to produce pulps from pines of 5460 G.E. brightness in the 5260% yield range. These pulps have strength qualities markedly superior to the sulphite pulp produced from sulphur-dioxide fortified magnesium base sulphite liquor. In the attached drawings there is illustrated a flow diagram of a pulping cycle.

One of the advantages of this process in comparison with the processes such as acid sulphite process is that it is possible to raise the temperature to substantially 188 C. In the acid sulphite process it is best to operate within a maximum of about 140 C. to avoid excessive degradation of the cellulose.

I have discovered also that the cooking liquor can be at a temperature between about 70 C. and C. when being charged to the digester. As stated heretofore it has been considered desirable and in fact essential that in making sulphite pulp that the initial cooking liquor have a free S0 considerably greater than the combined S0 so that a large excess of sulphur dioxide be present and that a relatively low temperature be employed. For example, with a total S0 of 300500% or even more in excess of the combined S0 a temperature rise to C. to C. or less is normally considered to be essential. It has been discovered that starting with the magnesium bisulphite liquor with little or no excess sulphur dioxide above that required for a pure magnesium bisulphite solution that after complete impregnation of the chip the temperature can be raised quite rapidly to as much as about 180 C.

The most essential factor of this invention is that the cooking liquor should completely impregnate pine chips before higher temperatures are attained, that is, temperatures above 100 to 110 C. Steaming before impregnation of the liquor into the chips must not be done because this promotes decomposition of the wood whereby wood acids are formed which promotes the previously described undesirable lignin-stilbenes reaction.

To prevent the reaction of the pinosylvin and p'inosylvin monomethylether with the lignin of the heartwood the cooking liquor must penetrate into the cell and cell walls in the entire chip including the middle of the chips before higher temperatures are reached which cause formation of wood acids. The production of wood acids in the wood cells and cell walls before complete impregnation of the Wood with magnesium bisulphite results in the reaction of the pinosylvin and pinosylvin monomethylether with the heartwood lignin. After the cooking liquor has substantially completely impregnated the chips the temperature can be raised rapidly to a temperature of the order of C. to 188 C.

In general, the process of the present invention comprises impregnating the chips with the magnesium bisulphite cooking liquor at a relatively low temperature. The temperature of the chips should not exceed about 100 C. to 110 C. until completely impregnated. Thereafter the completely impregnated chips are then heated to the desired temperature between 155 C. and 188 C. This elevated temperature is held for a predetermined period of time depending on the temperature and pressure employed. At 163 C. this may be in the order of one and one-half hours and at 188 C. may be shortened to about one-half to one hour when the digester pressure is held at 70 to 100 p.s.i. to give a pine pulp having a yield of about 55% to about 65%, oven dry pulp from oven dry chip basis.

The chips are supplied to a suitable digester and are not presteamed. The magnesium .bisulphite. solution previously prepared and preheated is pumped to the digester containing the chips until full, the pumping being continued to build up a pressure of about 4 to 6 atmospheres or higher and held at that pressure for one to two hours to assist penetration of the chips. The surplus cooking liquor is then drained from the digester to give the desired liquor to wood ratio and returned to an accumulator from which it was drawn.

Excess of liquor for the purpose of this application is defined as the liquor volume over that required to cook the wood to the desired degree.

No free S as used in this application is defined as substantially no sulphur dioxide in excess of the stoichiometric amount necessary to form the bisulphite.

The temperature of the contents of the digester is brought up to not to exceed 100 C. to 110 C. in the one hour to two hours during the pressure penetration period by which time the chips are substantially completely impregnated with the cooking liquor. After impregnation the temperature is brought up to the value desired for cooking, for example 163 C. in one to four hours. The temperature is held at this value for the desired time, for about one-half to about one and one-half hours, for example one hour. The pressure in the meanwhile is held at a value slightly greater than that for the vapor pressure of pure water at the digester temperature which is about 90 p.s.i.g. at 163 C. The pressure of the digester is then relieved and the pressure and the temperature dropped from about 90 p.s.i.g. at 163 C. to about 153 C. at 60 p.s.i.g. The contents of the digester are then discharged to a blow tank. The pulp in the blow tank is suspended in a portion of the residual liquor and passes through suitable washers. The washed pulp may be subjected to mechanical refining, screening, cleaning and/ or bleaching and made into paper. The temperature of the cooking liquor going to the digester is usually within the range of about 80 C. to about 110 C. The pH of the liquor going to the digester being within the range of about 3.5 to 5.5. The pH of a cooled liquor sample from the digester at the end of the cook is about 2.5 to 4.0.

The ratio of chips to cooking liquor after withdrawing the excess liquor may fall within the range of to 1 and under certain conditions to about 2 to 1.

Example 1 One hundred and twenty-eight pounds of chips, oven dry basis, were placed in a digester. A magnesium bisulphite liquor at a temperature of about 70 C. having a pH of 4 containing substantially equal parts of free S0 and combined S0 (about 2.35% of each) was added to cover the chips; a hydrostatic pressure of about 90 p.s.i.g. was applied for one and one-half hours and the temperature of about 100 C. reached in the chips. The excess liquor was withdrawn to leave a residual liquor to wood ratio of 4.0 to 1. The temperature was brought up to 163 C. in two and one-fourth hours. The temperature was held at 163 C. for one and one-quarter hours. The digester was relieved from 90 p.s.i.g. to 60 p.s.i.g. and the temperature dropped to 153 C. The period of relief from 90 p.s.i.g. to 60 p.s.i.g. was one and one-half hours. The resulting cooked material was washed, refined and screened. The yield was 5 6.8%, oven dry pulp from oven dry chips basis, the permanganate number was 51 (TAPPI 40 cc. basis, A gram sample x 4) and the GE. brightness was 54%.

Example 2 One hundred and twenty-eight pounds of chips, oven dry basis, were placed in a digester. A magnesium bisulphite liquor at a temperature of about 70 C., having a pH of 4 containing substantially equal parts of free S0 and combined S0 (about 2.35% of each) was added to cover the chips; a hydrostatic pressure of about 90 p.s.i.g. was applied for one and one-half hours and the temperature of about 100 C. reached in the chips. The excess liquor was withdrawn to leave a residual liquor to wood ratio of 4.0 to 1. The temperature was brought up to 158 C. in two and one-fourth hours. The temperature was held at 158 C. for one and one-quarter hours. The digester was relieved from p.s.i.g. to 60 p.s.i.g. and the temperature dropped to 153 C. The period of relief from 90 p.s.i.g. to 60 p.s.i.g. was one and one-half hours. The resulting cooked material was washed, refined and screened. The yield was 60.5%, oven dry pulp from oven dry chips basis, the permanganate number was 61 (TAPPI 40 cc. basis, A gram sample x 4) and the GE. brightness was 53%.

Example 3 One hundred and twenty-eight pounds of chips, oven dry basis, were placed in a digester. A magnesium bisulphi-te liquor at a temperature of about 70 C. having a pH of 4 containing substantially equal parts of free S0 and combined S0 (about 2.35% of each) was added to cover the chips; a hydrostatic pressure of about p.s.i.g. was applied for one and one-half hours and the temperature of about C. reached in the chips. The excess liquor was withdrawn to leave a residual liquor to wood ratio of 4.0 to 1. The temperature was brought up to 168 C. in two and one-fourth hours. The temperature was held at 168 C. for one and one-quarter hours. The digester was relieved from 95 p.s.i.g. to 75 p.s.i.g. and the temperature dropped to 157 C. The period of relief from 90 p.s.i.g. to 60 p.s.i.g. was one and one-quarter hours. The resulting cooked material was washed, refined and screened. The yield was 52.8% oven dry pulp from oven d-ry chips basis, the permanganate number was 41 (TAPPI 40 cc. basis, A gram sample x 4) and the G.E. brightness was 56%.

To increase the GE. brightness of the pulp by one or two points there shall remain a small amount of excess S0 at the end of the digestion period. An amount of the order of about two grams per liter will give the additional brightness. It is preferred that the surplus S0 be added to the digester in the pulping cycle downstream from the time the maximum temperature has been applied for a predetermined time and after lowering of the pressure.

Unbleached bisulphite produced in accordance with this process at about 60% yield has high tear, tensile and unbleached brightness, good formation and rapid development to give maximum strength at high freeness level. The pulp produced by this process contains screenable rejects at about .2 to about 2% and has a tensile breaking length in excess of about 13,500 meters as measured after beating to 300 cc. Canadian freeness.

About two grams per liter of S0 may be added after decreasing the pressure to at least about 70 p.s.i. and thereafter continuing the cooking at a temperature in excess of about 150 C. for ten minutes to about 60 minutes.

I claim:

1. The method of preparing pulp from pine wood chips comprising charging the digester with pine wood chips, flooding the digester with an excess of magnesium bisulphite liquor at a temperature of about 70 C. to about C. and having a pH of about 2.5 to about 4.0 and containing substantially equal parts of free S0 and combined S0 the temperature of the chips not exceeding about 110 C. during impregnation, withdrawing the excess cooking liquor and raising the temperature of the impregnated chips to about 163 C. for a predetermined time, holding the temperature at about 163 C. for a predetermined time and then lowering the temperature to about 153 C. adding sulphuric acid and continuing the cook for a predetermined time of about 10 minutes to about 60 minutes with the temperature being in excess of C.

2. The process of preparing pulp from pine wood chips comprising charging the digester with pine wood chips, adding magnesium bisulphite liquor at a temperature of about 70 C. and having a pH of about 4, said cooking liquor containing substantially equal parts of free S0 and combined S0 raising the pressure of the digester to about 90 pounds per square inch, applying a temperature of about 100 C. for about one and one-half hours to the digester contents, removing the excess liquor to leave a residual liquor to wood ratio of about 4.0 to about 1, raising the temperature to about 163 C. in about two and one-fourth hours, holding the temperature at about 163 for about one and one and one-fourth hours, then relieving the pressure, dropping the temperature to about 153 C. and then removing the cooked material from the digester and subjecting it to mechanical refining.

3. A method of preparing pulp from pine wood chips comprising charging a digester with pine wood chips, flooding the chips with a magnesium bisulphite liquor having a pH of about 2.5 to about 5.5, said cooking liquor containing substantially equal parts of free S0 and combined S0 raising the pressure of the digester to about 90 pounds per square inch and applying the pressure for about one and one-half hours at a temperature not exceeding 110 C., removing the excess liquor to leave a residual liquor to wood ratio of about 4.0 to 1, then raising the temperature of the impregnated chips to about 158 C. in about two and one-fourth hours, holding the temperature at 158 C. for about one and one-fourth hours, then relieving the digester to about 60 pounds per square inch and dropping the temperature to about 150 C., adding sulphur dioxide to the digester and continuing the cooking for about one-half hour at a temperature not less than about 150 C.

4. A method of preparing pulp from pine wood chips comprising charging chips to a digester, adding magnesium bisulphite at a temperature within the range of about 70 to 110 C. and having a pH of about 2.4 to about 5.5, said liquor containing substantially equal parts of free S0 and combined S0 the liquor added to the digester until the chips are completely covered and raising the pressure of the digester to at least 70 pounds per square inch, and holding this pressure for one and onehalf hours and raising the temperature not to exceed 110 C., withdrawing the excess liquor, raising the temperature of impregnated chips to 168 in about two and one-fourth hours and holding the impregnated chips at this temperature for about one to about two hours and relieving the digester to lower the pressure and drop the temperature to not less than 150 C. and then removing the cooked material from the digester.

5. A method of preparing pulp from pine Wood chips comprising charging a digester with pine wood chips, flooding the chips with a hot magnesium bisulphite liquor having substantially equal parts of free S0 and combined S0 applying a pressure in excess of 70 p.s.i. and a temperature such that the chips do not exceed 110 C. during substantial complete impregnation, withdrawing the excess liquor and raising the temperature to about 155 C. to about 188 C. in about two to about three hours, holding this temperature for a predetermined time of about one-half to about two and one-half hours with the solution having a pH of about 2.5 to about 2.4 at the completion of digestion as measured at room temperature.

6. A method of preparing pine wood chips comprising charging a digester with pine wood chips, flooding the chips with hot magnesium bisulphite liquor having substantially equal parts of free S0 and combined S0 applying pressure in excess of 70 p.s.i. and increasing the temperature to within a range of about 90 C. to less than about 110 C. and substantially completely impregnating the chips, withdrawing the excess of liquor, raising the temperature to about 155 C. to about 188 C. and maintaining the temperature for a predetermined time about one and one-fourth hours, lowering the pressure and temperature, adding sulphur dioxide, increasing the pressure to about p.s.i. to about p.s.i. and continuing the cooking at a temperature in excess of about C. for ten minutes to about 60 minutes.

7. The process of preparing pulp from pinewood chips consisting of charging a digester with pinewood chips, flooding digester with magnesium bisulphite liquor at a temperature of about 70 'C. to about 100 C. and having a pH of about 2.5 to about 4 and containing about 2.35% free S0 and about 2.35% combined S0 applying a hydrostatic pressure of about 90 p.s.i.g. on the digester contents for about one and one-half hours, holding the temperature in the digester at about 100 C., withdrawing the excess bisulphite liquor to leave a residual liquor to wood ratio of about 1.0 to 4.0, rapidly bringing the digester temperature up to about 163 C. in about two and one-fourth hours, holding the digester temperature at about 163 C. for about one and one-fourth hours, lowering the digester pressure from 90 p.s.i.g. to 60 p.s.i.g. and the temperature to about 153 C. in one and one-half hours and then discharging the digested chips from the digester.

8. The process of preparing pulp from pinewood chips consisting of charging a digester with raw pinewood chips, flooding digester with magnesium bisulphite liquor at a temperature from 70 C. to about 100 C. and having a pH of about 2.5 to about 4.0 and containing about 2.35% free S0 and about 2.35 combined S0 applying a pressure of about 90 p.s.i.g. on the digester for about one and one-half hours, holding the temperature at about 100 C. for about 1 to about 2 hours, withdrawing the excess liquor to leave a residual liquor to wood ratio of about 1.0 to 4.0, rapidly bringing the digester temperature to about 158 C. to about 168 C. in about two and onefourth hours, holding the digester temperature at about 158 C. to about 168 C. for about one and one-fourth hours, lowering the digester pressure and then discharging the digested chips.

References Cited by the Examiner UNITED STATES PATENTS 2,885,317 5/59 Collin 162-84 3,046,182 7/62 Tomlinson 162-83 3,067,086 12/62 Wilson 16283 OTHER REFERENCES Bailey: Recent Technical Advances in Magnesia-Base gooking, TAPPI, pp. 689691, September 1962, vol. 45,

Casey: Pulp & Paper, pp. -166, 170, 171, 2nd edition, vol. 1, pub. by Interscience Pub., N.Y., NY. (1960).

.McGovern: Pulping Loblolly Pine Heartwood, Paper Trade Journal, pp. 32-35, October 16, 1941.

Soderquist: New Process for Successful Production of Sulphite Pulp from Pine, Paper Trade Journal, pp. 30-36, Oct. 17,1955.

DONALL H. SYLVESTER, Primary Examiner. HOWARD R. CAINE, Examiner. 

1. THE METHOD OF PREPARING PULP FROM PINE WOOD CHIPS COMPRISING CHARGING THE DIGESTER WITH PINE WOOD CHIPS, FLOODING THE DIGESTER WITH AN EXCESS OF MAGNESIUM BISULPHITE LIQUOR AT A TEMPERATURE OF ABOUT 70*C. TO ABOUT 110*C. AND HAVING A PH OF ABOUT 2.5 TO ABOUT 4.0 AND CONTAINING SUBSTANTIALLY EQUAL PARTS OF FREE SO2 AND COMBINED SO2, THE TEMPERATURE OF THE CHIPS NOT EXCEEDING ABOUT 110*C. DURING IMPREGNATION, WITHDRAWING THE EXCESS COOKING LIQUOR AND RAISING THE TEMPERATURE OF THE IMPREGNATED CHIPS TO ABOUT 163*C. FOR A PREDETERMINED TIME, HOLDING THE TEMPERATURE AT ABOUT 163*C. FOR A PREDETERMINED TIME AND THEN LOWERING THE TEMPERATURE TO ABOUT 153*C. ADDING SULPHURIC ACID AND CONTINUING THE COOK FOR A PREDETERMINED TIME OF ABOUT 10 MINUTES TO ABOUT 60 MINUTES WITH THE TEMPERATURE BEING IN EXCESS OF 150*C. 